With the recent rapid development of portable and cordless electronic devices such as audio-visual (AV) devices and personal computers, there is an increasing demand for secondary batteries or batteries having a small size, a light weight and a high energy density as a power source for driving these electronic devices. Also, in consideration of global environments, electric vehicles and hybrid electric vehicles have been recently developed and put into practice, so that there is an increasing demand for lithium ion secondary batteries for large size applications having excellent storage characteristics. Under these circumstances, lithium ion secondary batteries having advantages such as large charge/discharge capacity and good storage characteristics have been noticed.
Hitherto, as positive electrode active materials useful for high energy-type lithium ion secondary batteries exhibiting a 4 V-grade voltage, there are generally known LiMn2O4 having a spinel structure, LiMnO2 having a zigzag layer structure, LiCoO2 and LiNiO2 having a layer rock-salt structure, or the like. Among the secondary batteries using these active materials, lithium ion secondary batteries using LiNiO2 have been noticed because of large charge/discharge capacity thereof. However, the materials tend to be deteriorated in thermal stability under a charged condition and charge/discharge cycle, and, therefore, it has been required to further improve properties thereof.
Specifically, when lithium ions are released from LiNiO2, the crystal structure of LiNiO2 suffers from Jahn-Teller distortion since Ni3+ is converted into Ni4+. When the amount of Li released reaches 0.45, the crystal structure of such a lithium-released region of LiNiO2 is transformed from hexagonal system into monoclinic system, and a further release of lithium therefrom causes transformation of the crystal structure from monoclinic system into hexagonal system. Therefore, when the charge/discharge reaction is repeated, the crystal structure of LiNiO2 tends to become unstable, so that the resulting secondary battery tends to suffer from poor cycle characteristics and reaction between LiNiO2 and an electrolyte solution owing to release of oxygen therefrom, resulting in deterioration in thermal stability and storage characteristics of the battery. To solve these problems, there have been made studies on materials formed by adding Co and Al to a part of Ni of LiNiO2. However, these materials have still failed to solve the above-described problems. Therefore, it has still been required to provide a Li—Ni composite oxide having a more stabilized crystal structure.
Further, in the process for producing the Li—Ni composite oxide, in order to obtain the Li—Ni composite oxide having a high packing property and a stable crystal structure, it is required to use Ni composite hydroxide particles which are well controlled in properties, crystallinity and contents of impurities, and calcine the particles under the condition which is free from inclusion of Ni2+ into Li sites thereof.
More specifically, it is required to provide Li—Ni composite oxide capable of exhibiting a high packing property, a stable crystal structure and an excellent thermal stability under a charged condition as a positive electrode active material for a non-aqueous electrolyte secondary battery.
Hitherto, in order to improve various properties such as stabilization of a crystal structure and charge/discharge cycle characteristics, various improvements of LiNiO2 particles have been attempted. For example, there is known the technique of coating the surface of LiNiO2 with a Li—Ni—Co—Mn composite oxide to improve cycle characteristics and thermal stability thereof (Patent Document 1). Also, there are known the technique of mixing a Li—Co composite oxide and a Li—Ni—Co—Mn composite oxide with each other to improve charge/discharge cycle characteristics and thermal stability of the Li—Co composite oxide (Patent Document 2); the technique of suspending lithium carbonate, Ni(OH)2, Co(OH)2 and manganese carbonate in a Li—Co composite oxide, or by mechanically treating and coating the Li—Co composite oxide with a Li—Ni—Co—Mn composite oxide to improve charge/discharge cycle characteristics and high-temperature characteristics of the Li—Co composite oxide (Patent Documents 3 and 4); or the like, although these techniques are different in kind of material from those relating to the Li—Ni composite oxide.
Patent Document 1: Japanese Patent Application Laid-open (KOKAI) No. 2004-127694
Patent Document 2. Japanese Patent Application Laid-open (KOKAI) No. 2005-317499
Patent Document 3: Japanese Patent Application Laid-open (KOKAI) No. 2006-331943
Patent Document 4: Japanese Patent Application Laid-open (KOKAI) No. 2007-48711